The invention relates generally to a method and system for time-multiplexing a single VOR (VHF Omni-directional Range) receiver, and more specifically to time-multiplexing a single VOR receiver to determine bearings from two or more VOR stations.
Very high frequency omni-directional range (VOR) radio navigation has existed since the 1940s. The basic function of a VOR navigation system is to provide an aircraft with a means for determining direction by referencing a ground-based VOR transmitter station or for determining relative position by referencing multiple VOR stations.
A VOR transmitter modulates two reference signals onto a particular carrier frequency assigned to that VOR station (VOR carrier frequencies exist between 108.00 MHz and 117.95 MHz). The first reference signal is a pure 30 Hz AM tone and the second reference signal is a 30 Hz tone that is FM modulated at 9,960 Hz with a deviation ratio of 16. One of these signals is transmitted so that its phase is constant when measured in any direction from the VOR station. The other signal will show a relative phase shift between 0 and 360 degrees depending upon the bearing of the aircraft from the VOR station. The variable phase signal is aligned so that there is no phase shift when measured from a bearing of magnetic north from the ground station where the VOR transmitter is located. Depending upon the transmitter design, either 30 Hz signal may be used as the variable phase signal. The VOR receiver carried by an aircraft determines the phase differential between these two signals and thus with reference to magnetic north provides the pilot with an indication of his current bearing from the VOR station. For instance, if the phase differential is 128 degrees, then the aircraft is at a bearing of 128 degrees from that VOR station.
Taken together, these VOR stations are the basis for an extensive network of airways used by air traffic for en-route navigation. Aircraft fly along paths defined by radials from VOR stations and use onboard VOR receivers to track the radials defining the airways. Typically, aircraft will carry two VOR receivers which, of course increases the cost, weight, and size of these types of VOR navigation systems. One VOR receiver is used to track the current course to or from a VOR station, while the second VOR receiver can be used to determine the aircraft""s bearing from other VOR stations. These additional bearings from the other VOR stations can then be used to determine a two-dimensional relative position fix for the aircraft at the point where the bearings intersect.
Current VOR navigation systems that use one VOR receiver do not receive, process, or display more than one VOR signal at any given time. In order to receive both a primary VOR signal to follow or a localizer signal when near a runway, and a standby VOR signal for intersection calculations, a user of currently existing single receiver VOR navigation systems must constantly, manually swap back and forth between two or more VOR stations transmitting on different carrier frequencies and wait for signal acquisition.
This delay in honing in on the carrier frequency is because of the need to keep the received signal at a stable level; all radio receivers have some kind of automatic gain control (AGC) to stabilize the signal level. Traditional VOR receivers use a relatively slow integrator loop to control the gain stages of the amplifier. A slow AGC is necessary so the 30 Hz tone signals will not be washed out. Again, the phases of the 30 Hz signals must be compared in order to compute their phase differential and thus the bearing radial. The slow integrator loop makes it impossible to rapidly tune the receiver to multiple VOR stations on different frequencies that may have drastically different signal levels.
There is a need for a single receiver VOR navigation system that can receive and display multiple VOR signals and a localizer signal without user intervention. Thus, there is also a need for an automatic gain control to maintain acceptable signal strengths and operate quickly and accurately.
The present invention provides a single receiver VOR navigation system that can receive and display multiple VOR signals without user intervention by using a digital signal processor (DSP) to perform quick and accurate automatic gain control by controlling the amplifier stages to maintain an acceptable signal strength.
Each neighboring VOR station transmits on different carrier frequencies. The amplification of the 30 Hz signals used to determine bearing are based on the strength of the carrier frequency. The present invention allows for the multiplexing of a single VOR receiver to receive additional VOR stations"" carrier frequencies. The multiplexing is facilitated by first using stored AGC parameter settings for each frequency to quickly set the amplifier stage for another VOR station that has different path characteristics. To allow for the selection of multiple carrier frequencies a digital synthesizer is used to tune an oscillator. This type of digital tuner can be adjusted in precise sequence with the digital signal processing of the automatic gain control to develop a VOR multiplexing system or VOR Monitor. A localizer signal which is in the same band as the VOR signals may also be multiplexed by the system if selected by the user.
By using a combination of hardware and software, the DSP can control and adjust the amplifier gain stages and thus the total signal amplification may be changed from a minimum level to a maximum level (or anywhere in between) in an extremely short period of time. Additionally, the DSP software can save the amplifier settings used for a particular VOR frequency or localizer frequency to be used as a close starting point when the frequency is shortly re-selected.
According to one embodiment of the present invention, there is disclosed a method for determining a relative position of an object using multiple frequencies received from multiple transmitter locations which includes receiving a first frequency transmitted from a first known transmitter location using a receiver, determining a first radial bearing for the object relative to the first known transmitter location from the first frequency, receiving a second frequency transmitted from a second known transmitter location wherein the second frequency is received using the receiver used to receive the first frequency and without requiring a user to manually change a setting of the receiver to receive the second frequency, determining a second radial bearing for the object relative to the second known transmitter location from the second frequency, and using the first radial bearing and second radial bearing to determine the relative position of the object.
According to another embodiment of the present invention, there is disclosed a method for determining relative position of an object using multiple frequencies received from multiple transmitter locations using a receiver which includes selecting a first frequency transmitted from a first known transmitter location, retrieving automatic gain control parameters associated with the first frequency, receiving the first frequency using the automatic gain control parameters associated with the first frequency, determining a first radial bearing for the object relative to the first known transmitter location from the first frequency, selecting a second frequency transmitted from a second known transmitter location, retrieving automatic gain control parameters associated with the second frequency, receiving the second frequency using the automatic gain control parameters associated with the second frequency wherein the second frequency is received using the receiver used to receive the first frequency and without requiring a user to manually change a setting of the receiver to receive the second frequency, determining a second radial bearing for the object relative to the second known transmitter location from the second frequency, and using the first radial bearing and second radial bearing to determine the relative position of the object.
According to another embodiment of the present invention, there is disclosed a system for determining relative bearings of an object using multiple frequencies received from multiple transmitter locations which includes a receiver capable of receiving a plurality of frequencies and demodulating a plurality of signals, an amplifier stage, operative to adjust a signal strength of the plurality of signals in accordance with at least one automatic gain control parameter associated with each of the plurality of frequencies, an analog/digital converter, operative to convert the plurality of signals from a plurality of analog waveforms to a plurality of digital waveforms, one or more processors, operative to individually average the signal strength of each of the plurality of digital waveforms, to adjust the at least one automatic gain control parameter associated with each of the plurality of frequencies, to receive when an alternate frequency should be monitored and coupled to the receiver to adjust the receiver to receive another of the plurality of frequencies, one or more memory devices coupled to one or more processors and operative to store the at least one automatic gain control parameter associated with the plurality of frequencies and to, and store instructions for one or more processors.
According to another embodiment of the present invention, there is disclosed a system for processing multiple frequencies received from multiple transmitter locations which includes a receiver capable of receiving a plurality of frequencies and demodulating a plurality of signals, an amplifier stage, operative to adjust a signal strength of the plurality of signals in accordance with at least one automatic gain control parameter associated with each of the plurality of frequencies, an analog/digital converter, operative to convert the plurality of signals from a plurality of analog waveforms to a plurality of digital waveforms, one or more processors, operative to individually average the signal strength of each of the plurality of digital waveforms, to adjust the at least one automatic gain control parameter associated with each of the plurality of frequencies, and coupled to the receiver to adjust the receiver to receive another of the plurality of frequencies, and one or more memory devices coupled to the digital signal processor and operative to store the at least one automatic gain control parameter associated with the plurality of frequencies.